Fuel tanks comprise a fuel level detector to determine the volume remaining in the tank. The level detector can also be coupled to a fuel concentration detector. Each detection can be performed, for example, acoustically. An ultrasound emitter can thus generate waves that are reflected by the upper surface of the fuel then captured by an ultrasound receiver that makes it possible to determine, depending on the time elapsed between emission and receipt, the height of the fuel in the tank. A similar principle also makes it possible to determine the variations of concentration from a constant wave path.
These detectors use considerable electrical energy and involve the use of an emitter/receiver assembly which increases not only the weight of the tank, and incidentally of the motor vehicle, but also the cost of the components.